Disordered magnetism at the grain boundary of pure nanocrystalline iron

Nanocrystalline iron was synthesized by two different processing techniques in order to study the influence of the magnetic properties of grains boundaries on the macroscopic magnetic behavior. First, nanocrystalline Fe powders with an average crystallite size of around 12 nm were obtained by high-energy ball milling. Two different components are resolved by Mossbauer spectroscopy, which are attributed to the crystallites and to the grain-boundary region. The results of dc and ac susceptibility, remanence and magnetic relaxation measurements indicate a transition from a high-temperature ferromagnetic state to a low-temperature disordered regimen. The effect has been connected with the order-disorder transition of the spin located at the grain boundary. Secondly, we report on transport and magnetic measurements on Fe thin films grown by sputtering. A surprising minimum in the low-temperature thermal dependence of the resistance is observed. Since such anomaly completely disappears under the action of a magnetic field it is inferred that it originates from spin-dependent scattering arising from the disordered grain boundaries. All results clearly indicate that below a critical temperature, both systems behave as a random assembly of weakly interacting magnetic units.